feat(pwa): installable app + fix research tree pip overflow

Research tree UI:
- The level pips shared the header row with the upgrade name and fought
  for width; long names like "Festungsmauern" (5 pips) pushed the pips
  out of the upgrade card. Pips now sit in the bottom row next to the
  buy button (space-between), where width is plentiful, and carry a
  "level / max" tooltip
- Tighter modal padding on phones (max-width 560px), scrollable at 92vh

PWA (installable as app in mobile browsers):
- public/manifest.webmanifest: standalone display, any orientation,
  TRXTD theme colors, German lang, start_url "/"
- public/sw.js: network-first service worker with runtime caching.
  index.html is always fetched fresh so new deploys are picked up
  immediately; the cache only serves as offline fallback. API calls
  and cross-origin requests are never intercepted
- Icon set generated by scripts/generate-icons.mjs (pure Node PNG
  encoder, zero image dependencies): gold tower on the game-themed
  dark gradient in 192px, 512px, maskable 512px (motif inside the safe
  zone) and 180px apple-touch-icon
- index.html: manifest link, favicon, apple-touch-icon, theme-color,
  mobile-web-app-capable, apple-mobile-web-app meta tags
- src/main.ts: service worker registration in production builds only
  (dev HMR stays untouched)
- server.mjs: serve .webmanifest as application/manifest+json (Chrome
  requires the correct MIME type for installability)

Verified against the production server: manifest (application/
manifest+json), icons (image/png) and sw.js (text/javascript) respond
with 200, index.html references all PWA tags; npm test 48/48 green.

public/manifest.webmanifest

@@ -0,0 +1,32 @@
+{
+  "name": "TRXTD – Tower Defense",
+  "short_name": "TRXTD",
+  "description": "Tower Defense im Browser – verteidige deine Basis gegen 20 Wellen!",
+  "lang": "de",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "any",
+  "background_color": "#0e131a",
+  "theme_color": "#0e131a",
+  "icons": [
+    {
+      "src": "/icons/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/icons/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/icons/icon-maskable-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
+    }
+  ]
+}